Regulated power supplies provide a stable output voltage. Many applications exist for regulated power supplies. Personal computers and printers provide two exemplary examples. One such regulated power supply is commonly known as the buck-boost switch mode regulator as shown in prior art FIG. 1 which generates a stable output voltage from a supply voltage that may be lower or higher than the output voltage being regulated. This is traditionally accomplished using conventional switch mode regulation techniques which require an error amplifier and ramp generator.
In the buck-boost switch mode regulator of prior art FIG. 1, an error amplifier 1 compares the output voltage against an internal reference voltage VREF. An on chip ramp generator provides a ramp signal that typically may between 0 to 3 volts to an input of a comparator 2. Adjustment of the switching threshold of the error amplifier is accomplished with R1 and R2. A compensation resistor R and capacitors C and C2 define the time constant of the error amplifier. A connection pin typically exists for making external connection to the error amplifier. Capacitor C is usually located off chip due to its relatively large size of (typically several thousand picofarads). Locating resistor R off chip also aids in keeping its resistance constant over temperature changes. Resistor R typically has a value of 2 to 3 Kohms. Capacitor C2 is usually small enough so that is located on chip. Transistors Q1 and Q2 are power transistors and may be of the lateral drain metal oxide semiconductor (LDMOS) type. They are typically located off chip although they may be on chip. Transistor Q1 is the high side switching transistor and transistor Q2 is the low side switching transistor. Schottky diodes S1 and S2, storage capacitor C1, and resistors R1 and R2 are typically off chip.
Buck regulators (where the supply voltage must always be higher than the output voltage) are available in both conventional configurations as shown in prior art FIG. 2 and in burst mode configuration as shown in prior art. FIG. 3. The conventional way to detect switching thresholds for the burst type buck regulator is to detect the output voltage rising and turn off the switching transistor Q1 once the threshold has been reached. FIG. 3 accomplishes this by adding flip flop 3 which is reset by a periodic reset pulse having the same frequency as the ramp generator of FIGS. 1 and 2. The switching transistor Q1 is switched back on, following a periodic reset pulse from a pulse generator, providing that the output voltage has fallen below the output voltage threshold. A voltage divider (formed by feed-forward capacitor C3, resistors R1 and R2) reduces the output voltage so that it may be compared to the level of VREF. The feed-forward capacitor C3 provides a low impedance path for any transient fast moving signals and may or may not be provided in the burst design.
While the burst type topology of prior art FIG. 3 is simpler to implement than the buck type topology of prior art FIG. 2 as it has only a comparator, no error amplifier and no ramp generator circuitry, it has the drawback of providing a larger output ripple voltage. However, unlike the buck-boost topology of prior art FIG. 1, the burst type topology of prior art FIG. 3 is unable to be utilized in directing a burst type buck-boost (where the input voltage may be smaller than the required output voltage) which may be particularly useful in printer applications, for example. What is needed, therefore, is a regulated power supply having the advantages of both the buck-boost topology and the burst topology.
It is accordingly an object of the invention to provide a regulated power supply having both buck-boost and burst characteristics.
Other objects and advantages of the invention will be apparent to those of ordinary skill in the art having the benefit of the specification and drawings herein.